1. Field of the Invention
The invention relates to an information recording and reproducing apparatus for recording or reproducing information onto/from an information recording medium such as magnetic disc, magnetooptic disc, or the like and, more particularly, to a velocity control apparatus used during a seeking operation of a head which is used to record or reproduce information.
2. Related Background Art
Hitherto, as a seeking method of moving an information recording or reproducing head to a target position on an information recording medium, a method whereby the velocity of the head is always monitored and the head is allowed to reach a target position while controlling the velocity of the head in accordance with a predetermined movement schedule is generally known. FIG. 1 is a diagram showing the relations among a reference velocity and an actual velocity in the above general velocity control method and a current applied to an actuator for driving the head. In the diagram, a reference velocity V.sub.ref denotes a velocity based on a movement schedule of the head and is calculated in accordance with the remaining distance to a target position. The reference velocity V.sub.ref is obtained by the following equation. EQU V.sub.ref =[2.multidot..alpha.(S-.lambda./2.multidot.N].sup.1/2( 1)
where,
S: target moving distance PA1 .alpha.: deceleration/acceleration PA1 N: count value on a zero-cross point after the start of the seeking operation
The actual velocity of the head is sequentially detected in order to allow the head velocity to follow the reference velocity. For signal is detected. A time .DELTA.t.sub.n which is required instance, the zero-cross point of a tracking error point is measured. A current velocity V.sub.n of the head from the zero-cross point to the next zero-cross is calculated from the time .DELTA.t.sub.n and a track pitch .lambda. in accordance with the following equation. EQU V.sub.n =.lambda./2.times.1/.DELTA.t.sub.n ( 2)
The distance between the zero-cross points is equal to 1/2 of the track pitch .lambda..
When the velocity of the head is controlled, a command value of an actuator is calculated every predetermined period from the current velocity and the target velocity at that time. The head velocity is controlled by the obtained command value. A command value A.sub.ct is calculated by the following equation. EQU A.sub.ct =K(V.sub.ref -V.sub.n) (3)
(K: feedback gain of the velocity control system)
According to the conventional method as mentioned above, the head velocity is always detected and the command value obtained from the detected velocity of the head and the target velocity is fed back every predetermined period. Due to this, the head is moved in conformity with the target velocity as shown in FIG. 1 and seeks to the target position in accordance with the predetermined movement schedule.
However, according to the above conventional velocity control method, since the head velocity is obtained only for every track crossing operation, in the case where a velocity control of a predetermined period is executed irrespective of the track crossing period, as shown in FIG. 2, for example, when the velocity control is performed at a point P.sub.c, the velocity obtained at a point P.sub.c ' is used, so that a time delay of .DELTA.t.sub.c occurs during such a duration. Even at points P.sub.b and P.sub.a, time delays of .DELTA.t.sub.b and .DELTA.t.sub.a also similarly occur. Therefore, the velocity at the point P.sub.c is set to c' and has a value different from the real velocity c. Likewise, the velocity at the point P.sub.b is set to b' for the real velocity b. The velocity at the point P.sub.a is also set to a' for the real velocity a. In such a conventional method, since a large error occurs in the head velocity due to a deviation between the detection timing of the velocity and the control timing, it is difficult to accurately control the seeking operation of the head.